1. Field of the Invention
Embodiments described herein relate generally to a pattern inspection method. For example, embodiments described herein relate generally to an inspection apparatus and method that inspect a pattern by acquiring an optical image of a pattern image through irradiation of laser light or an electron beam.
2. Related Art
In recent years, with increasingly higher degrees of integration and larger capacities of large-scale integration circuits (LSI), circuit line widths demanded for semiconductor devices become increasingly narrower. Such semiconductor devices are fabricated by exposing and transferring a pattern onto a wafer by step-and-repeat equipment, the so-called stepper, to form a circuit using an original pattern (also called a mask or reticle; hereinafter, collectively called a mask) in which a circuit pattern is formed. Thus, to manufacture a mask for transferring such a fine circuit pattern to a wafer, a pattern writing apparatus capable of writing a fine circuit pattern and using an electron beam is used. By using such a pattern writing apparatus, a pattern circuit may directly be written onto a wafer. Alternatively, the development of a laser beam writing apparatus that writes by using a laser beam, in addition to the electron beam, is attempted.
The improvement of yield is indispensable for manufacturing LSI requiring a large amount of manufacturing costs. However, as is typically shown by DRAM (Random Access Memory) of 1 gigabit or so, patterns constituting LSI are in transition from the order of submicron to that of nanometer. One of the major causes that decrease the yield is a pattern defect of the mask used when a superfine pattern is exposed and transferred onto a semiconductor wafer by photolithography technology. With increasingly finer LSI pattern dimensions formed on the semiconductor wafer in recent years, dimensions that need to be detected as pattern defects are also extremely small. Therefore, a pattern inspection apparatus that inspects for defects of a transfer mask used for manufacturing LSI needs to be made more precise.
As an inspection method, a method of conducting an inspection by comparing an optical image obtained by capturing an image of a pattern formed on a target object like a lithography mask in a predetermined magnification by using an enlarging optical system with design data or an optical image obtained by capturing an image of the same pattern on the target object is known. Examples of the pattern inspection method include the die to die inspection that compares optical image data obtained by capturing an image of the same pattern in different places on the same mask and the die to database inspection that compares design image data (reference image) generated by inputting into an inspection apparatus pattern writing data (design pattern data) converted into the apparatus input format to be input into a pattern writing apparatus when a pattern is written using pattern-designed CAD data as a mask and an optical image to be measured data obtained by capturing the pattern. In such inspection methods of the inspection apparatus, a target object is placed on a stage and an inspection is conducted by scanning the target object with a luminous flux as the stage moves. The target object is irradiated with a luminous flux by a light source and an illumination optical system. Light transmitted through or reflected by the target object forms an image on a sensor via an optical system. The image captured by the sensor is sent to a comparator as measured data. After images are aligned, the comparator compares the measured data and reference data according to an appropriate algorithm and determines that there is a pattern defect if the measured data and the reference data do not match.
In such a pattern defect (shape defect) inspection, detection of pattern linewidth (CD) defects (CD error inspection) is also demanded. In the CD error inspection, the pattern linewidth in an obtained image is measured and a difference from design data is determined to check whether the difference of the linewidth is within a permitted value. Also, an inspection method that, instead of conducting a CD error inspection of each pattern for the entire mask surface, detects a linewidth defective region as a CD error by measuring the linewidth of a pattern in an image obtained for each set region, determining a difference from design data, and comparing the average value of differences of all linewidths in the region and a threshold is proposed (see Japanese Patent No. 3824542, for example).
With increasing micropatterning of late, micropatterning technology progresses in the pattern transfer using a mask. For example, micropatterning processes by double patterning and further, multiple patterning like triple, quadruple, . . . are increasingly used. In a micropatterning process based on superposition, the linewidth (CD) of a pattern itself formed on a mask does not become narrower even if the final transferred pattern becomes finer and thus, shape defects like irregularities of a pattern do not necessarily become smaller in proportion to micropatterning. Regarding the precision of the linewidth (CD), on the other hand, superposition precision is affected and thus, the permitted value of a CD error becomes smaller in proportion to micropatterning or multiplexing. Therefore, requirements for the CD error inspection become more stringent in proportion to micropatterning. However, the detection limit of a CD error by deep ultraviolet (DUV) light used as the inspection light is near its limit due to the pixel size and optical resolution.
The permitted value of a CD error becomes smaller, as described above, in proportion to micropatterning. However, as described above, the detection limit of a CD error by deep ultraviolet (DUV) light used for inspection light is near its limit. If CD of patterns of the entire mask surface is inspected by using a conventional inspection apparatus and making the determination threshold stricter (smaller), the number of patterns erroneously determined to be CD errors becomes huge, which makes the method an unrealistic inspection method. If the determination threshold is made smaller in an inspection method of detecting an abnormal linewidth region as a CD error, the number of regions erroneously determined to be abnormal linewidth regions becomes huge. If internal patterns of all regions determined to be CD errors should be checked, the amount of checks and the time needed for checks become huge, which makes the method an unrealistic inspection method. Thus, both methods have their limits to making the determination threshold smaller. Therefore, there exists a problem that it is difficult to conduct a high-precision CD error inspection accompanying increasing micropatterning.